The present invention relates to electronic flash apparatus utilized for photographing purposes.
As is well known, it is necessary to control the operation of an electronic flash apparatus so as to regulate the amount of flash illumination provided thereby so as to achieve a proper film exposure. In general, the extent of flash illumination provided by conventional electronic flash apparatus is determined according to two different types of systems, namely a parallel system or a series system. According to the parallel system, flash illumination is terminated by way of switching "ON" or closing a switching means connected in parallel with the flash discharge tube, while according to the series system termination of the flash illumination is made by turning "OFF" or opening a switching means connected in series with the flash discharge tube. With either of these systems, the switching means is controlled by way of analog types of control circuits. For example, a conventional arrangement will include an integrator circuit having a light-sensitive element receiving light reflected from the object, the circuit also including a capacitor connected in series with the light-sensitive element so that when the integrating operation of the circuit reaches a given value an output signal is produced which provides the ON or OFF control for the switching means.
Analog controls of the above type, however, are undesirably influenced by fluctuation of source voltage as well as variations in ambient temperature. In addition, the integrating time of the integrating circuit requires on the order of tens of microseconds, so that the output signal (integration signal) appearing as a voltage across the capacitor is highly unstable. For these reasons, it has been difficult with conventional apparatus to achieve accurate flash control.